The present invention claims priority from Japanese Patent Application No. 11-273186 filed Sep. 27, 1999, the contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a fault test of an IC (integrated circuit) and, particularly, to a determination of a fault of an IC based on a frequency spectrum analysis of a power source current.
2. Description of Related Art
In order to realize a high quality fault test and perform a fault test in a short time at low cost, it has been usual to detect a fault of an IC by analyzing a frequency spectrum of a power source current. For example, JP 11-142468 A discloses a technique in which an IC is determined as defective or acceptable by successively and repeatedly applying a unit test signal having a duration T to the IC and observing spectral powers of power source current flowing through the IC at a frequency 1/T corresponding to the duration T of the unit test signal and high harmonic frequencies which are integer multiples of the frequency 1/T. The disclosed technique is based on that, when the unit test signal is applied to an IC to be tested for the period T, the power source current having a constant waveform always flows for the period T.
However, depending upon an IC to be tested, there is a case where, even when the same test signal is inputted thereto, the same power source current having always the same waveform is not always flows. An example of such case is shown in FIG. 10, which explains the period of the waveform of power source current with respect to the unit test signal having the period T in a fault test of IC.
In FIG. 10, it is assumed that, when a unit test signal 1801 is applied to an IC under test, a state 1802 of the IC becomes a state A. In such case, the power source current 1803 becomes power source current A. When the same unit test signal is applied thereto secondly, the state 1802 of the IC becomes B, in which case the power source current 1803 flowing through the IC becomes B. With supply of a next unit test signal 1801 to the IC under test, the state returns to A and current A flows through the IC. Therefore, in the case where the state of the IC is alternated between A and B every time the unit test signal 1801 is supplied to the IC, the current flowing through the IC is changed between A and B. Consequently, the period of the power source current 1803 becomes twice the period T of the unit test signal, that is, 2T.
In a case where the state of the IC is changed from A through B to C repeatedly with supply of the unit test signals 1801 as shown in the state 1804, the period of the power source current 1805 becomes 3T.
In such conventional fault test of the IC, the spectral power at only frequency 1/T and high harmonics thereof are monitored. Therefore, an accurate test is difficult.
An object of the present invention is to provide a fault tester of IC, which is capable of detecting fault of IC accurately, a test method thereof and a recording medium recorded with a detection control program of the method.
The present invention is based on the fact that the reason why a constant power source current does not always flow through an IC under test even when identical unit test signals 1801 are supplied to the IC as shown in FIG. 10 is that the IC has a memory function and the power source current flowing through the IC according to the unit test signal depends upon the memory content of the IC and, therefore, even if the application periods of the unit test signal are constantly T, the period of the power source current flowing through the IC becomes an integer multiple of T. In such case, peaks of the frequency spectrum of the power source current appear at high harmonic frequencies, which are integer multiples of a basic frequency 1/nT where n is an integer. The present invention is featured by performing a test on the basis of this fact.
That is, according to a first aspect of the present invention, a fault tester for detecting a fault of an IC by monitoring power source current flowing through the IC when the latter is operated by test signals having a constant period T is provided, which is featured by comprising current detection means for monitoring a power source current supplied to the IC, spectrum analysis means for obtaining a frequency spectrum of the power source current detected by the current detection means and determination means for determining whether or not the IC has a fault on the basis of values of spectral power at frequencies in the spectrum, which are integer multiples of 1/nT, where n is an integer preliminary defined for the IC under test.
According to a second aspect of the present invention, a fault tester for detecting a fault of an IC by monitoring power source current flowing through the IC when the latter is operated by test signals having a constant period T is provided, which is featured by comprising current detection means for monitoring a power source current supplied to the IC, spectrum analysis means for obtaining a frequency spectrum of the power source current detected by the current detection means and determination means for determining whether or not the IC has a fault, the determination means being adapted, when a peak frequency in the frequency spectrum of the IC under test obtained by the spectrum analysis means is assumed as 1/nT, where n is an integer, to obtain an integer nxe2x80x2 representative of peak frequencies of respective power peaks of the frequency spectrum as frequencies integer multiples of 1/nxe2x80x2T and, when the integer nxe2x80x2 is different from the integer n, to determine the IC under test as having a fault. It is preferable that, when the integer nxe2x80x2 is equal to the integer n, a fault of the IC under test is determined on the basis of values of spectral power at frequencies in the spectrum, which are integer multiples of 1/nT.
According to a third aspect of the present invention, a fault detection method for detecting a fault of an IC by monitoring power source current flowing through the IC when the latter is operated by test signals having a constant period T is provided, which is featured by comprising the steps of monitoring a power source current supplied to the IC, obtaining a frequency spectrum of the power source current detected by the monitoring step and determining whether or not the IC has a fault on the basis of values of spectral power at frequencies in the spectrum, which are integer multiples of 1/nT, where n is an integer preliminary defined for the IC under test.
According to a fourth aspect of the present invention, a fault detection method for detecting a fault of an IC by monitoring power source current flowing through the IC when the latter is operated by test signals having a constant period T is provided, which is featured by comprising the steps of monitoring a power source current supplied to the IC, obtaining a frequency spectrum of the power source current detected by the monitoring step and obtaining an integer nxe2x80x2 representative of peak frequencies of respective power peaks in the frequency spectrum as frequencies integer multiples of 1/nxe2x80x2T when a peak frequency in the frequency spectrum of the IC under test obtained by the frequency spectrum is assumed as 1/nT, where n is an integer, and determining the IC as having a fault when the integer nxe2x80x2 is different from the integer n. It is preferable that, when the integer nxe2x80x2 is equal to the integer n, the method further comprises the step of determining a fault of the IC under test on the basis of values of spectral power at frequencies in the spectrum, which are integer multiples of 1/nT.